The standard for single-mode optical fibers, which are most widely used for optical transmission systems, has been laid down by International Telecommunication Union (ITU) as the G. 652 standard. The G. 652 standard defines conditions that have to be satisfied by the single-mode optical fibers, including respective ranges of the mode field diameter at a wavelength of 1310 nm, the cable cut-off wavelength, the zero dispersion wavelength, and the dispersion slope at the zero dispersion wavelength.
For optical transmission systems such as FTTH (fiber to the home), in which optical fibers are laid down to individual homes, and FTTC (fiber to the curb), in which optical fibers are laid down to curbs or utility poles, excess lengths of optical fibers have to be handled properly. To properly handle an excess length of an optical fiber, the excess length portion is wound and stored in a storage box. At this time, if the optical fiber has a small bending loss, the optical fiber can be wound with a small diameter, so that a small storage box can be used. Therefore, it is desirable that an optical fiber have a small bending loss. Japanese Unexamined Patent Application Publication No. 2007-140510 discloses an optical fiber that is compliant with the G. 652 standard and capable of considerably reducing the bending loss. However, this optical fiber is not prevented from being fractured when the optical fiber is accidentally bent during installation or other operations.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2007-140510